With increasing demand for high quality data services in modern wireless communications, it is more and more difficult to support required data capacity via conventional cell splitting techniques, which require additional macro evolved NodeBs (eNBs) for deployment. Actually, operators can make use of low power nodes, including remote radio heads, pico eNBs, home eNBs (HeNBs) and relay nodes, as a complement to enhance system coverage and capacity performance. And we refer to a network deployment incorporating one or more small-power nodes as mentioned above into the conventional macro eNB as a heterogeneous network (HetNet).
As one of the current hottest research topics, however, HetNet gives rise to strong interference problem in the overlapped coverage area between different nodes with co-channel deployment, which will seriously degrade the performance of victim user equipments (UEs) so that they are even unworkable in practice. The most severe interference scenarios in HetNets include two deployments in downlink (DL). The first is the Macro-HeNB deployment, shown in FIG. 1(a), wherein the MeNB denotes a macro eNB without the closed subscriber group (CSG) configuration, the HeNB denotes a home eNB with closed subscriber group (CSG) configuration, and the HUE denotes a UE served by the home eNB. In this deployment, the macro UE (MUE) being in close proximity of the HeNB and served by the macro eNB suffers the severe interference from the HeNB (since the restriction of CSG, the macro UE served by the macro eNB can not access to the HeNB via handover). The second is the Macro-PeNB deployment, shown in FIG. 1(b), where if the range expansion (RE) cell association with large bias is employed for enhancement of edge UE performance, load balancing, etc., the pico UE (PUE) served by the PeNB will suffer severe interference from signal transmitted by the macro eNB. Here the victim UE connects to the pico eNB, although at the viewpoint of the victim UE, the DL signal power from pico eNB is much lower than that from the macro eNB. The “aggressor eNB” means that its transmitted signal is regarded as the interference to the victim UE.
Moreover, several researches indicate that the interference problem of control channels (CCHs, including PDCCH, PCFICH, and PHICH) should be solved prior to that of the data channels because of the much more importance to guarantee signal reception. Besides, considering the inherent property of CCHs, it is almost impossible to use on the CCHs a resource block (RB)-level interference coordinated scheduling technique that copes with data channel interference problem conventionally.
Currently, considering the interference mitigation performance and backward compatibility to the legacy UEs, the most efficient existing solutions include almost blank subframe scheme and DL transmit power control scheme.
The basic idea of the almost blank subframe scheme is depicted in FIG. 2, where the “low power eNB” denotes HeNB or pico eNB. For interference mitigation, the aggressor eNB thoroughly mutes CCHs of certain DL subframes to avoid the time-frequency resource conflict to corresponding involved eNB and the victim UEs can only be scheduled into these specific subframes of serving eNB, e.g., the victim UEs served by the macro eNB are scheduled into SF 5 and the victim UEs served by low power eNB are scheduled into SF 3, respectively; while the non-victim UEs can share all other subframes of corresponding eNB for scheduling except the muted ones. Moreover, the data channels of these CCH-muted subframes are nulled either. It is noted that, however, these almost blank subframes can not be scheduled for the UEs served by aggressor eNB in any case. Therefore, this will result in a system performance loss, and it becomes more serious especially when the number of UEs served by the aggressor eNB is increasing.
According to certain criterion, the DL transmit power control scheme simply lowers the whole transmit power of aggressor eNB without subframe-specific consideration to avoid severe interference to the signal from serving eNB, depicted in FIG. 3. Obviously, the serving coverage of the power-adjusted aggressor eNB decreases accordingly. Therefore it can guarantee the workability of victim UEs to some extent, but there is much capacity performance loss of its normally served UEs inevitably.